<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>固本固布局</title>
    <style>
         .left {
            float: left;
            background: orange;
            height: 400px;
            width: 300px;
        }

        .right {
            float: right;
            background: blue;
            height: 500px;
            width: 300px;
        }
        .main {
            margin: 0 310px;
            height: 450px;
            background: green;
        }
    </style>
</head>

<body>
     <!-- 显示在左侧 -->
     <div class="left"></div>
     <!-- 显示在右侧 -->
     <div class="right"></div>
    <!-- 中间自适应宽度 -->
    <div class="main"></div>
   

</body>

</html>